Organic EL elements are surface light-emitting elements that utilize a phenomenon that light is emitted when an electric field is applied to a substance. Further, application of organic EL panels, in which an organic EL element is formed on a substrate, to planar light sources, displays, and the like is attempted by taking advantage of a feature that modification into a self-luminous type or a thin type is possible.
Currently, among organic EL panels, foldable (also referred to as flexible or bendable) organic EL panels are attracting attention. In order to realize a flexible organic EL panel, it is necessary to form an organic EL element on a substrate having flexibility. Further, since organic functional materials used for light-emitting layers, etc. of organic EL elements are apt to be deteriorated by moisture or oxygen, and this causes generation of dark spots or a decrease in luminous efficiency, it is necessary for a substrate to have a high barrier property against oxygen or moisture entering from outside of the element. In this regard, it is known that use of thin-plate glass as an element-forming substrate for organic EL elements can achieve both flexibility and a high barrier property (Patent Document 1).
However, thin-plate glass is easily broken, and a decrease in yield in a manufacturing process for forming an organic EL element on thin-plate glass is a problem. Thus, Patent Document 2 discloses a glass substrate for forming an organic EL element, in which a compressive stress layer is formed on the glass substrate by chemical strengthening in order to prevent cracking or chipping of thin-plate glass, and an alkali barrier film is formed in contact with the glass base material surface of the chemically strengthened glass base material. As the alkali barrier film, a film of SiO2 or SiNX formed by sputtering, CVD, or the like is exemplified.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2004-79432
Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2014-028743